1. Field of the Invention
The present invention relates to a zoom lens which can be advantageously used for an autofocus camera, particularly for a compact camera. More precisely, the present invention relates to a control method of a vari-focal lens including magnification control and focus control and a zoom lens barrel assembly to which the control method is applied.
2. Description of Background Information
A conventional zoom lens for a camera has a focal point which does not move during zooming. For instance, such a known zoom lens has a first lens group having a positive focal length and a second lens group having a negative focal length, wherein zooming is effected by varying the distance between the first and second lens groups, and focusing is effected only by the first lens group. However, in this type of zoom lens, it is necessary to increase the displacement of the lens groups in order to increase magnification. In practice, it is virtually impossible to obtain a large magnification because of the restricted displacement of the lens groups. A large magnification can be easily realized by a vari-focal lens. However, in a vari-focal lens, the focal point thereof moves during zooming, and accordingly, such a vari-focal lens is usually not used in a single-lens reflex camera.
In an autofocus camera having a photographic optical system and a separate finder optical system, the movement of the focal point which is caused by the zooming operation can be compensated for by the movement of the focusing lens group. Namely, since the position of the focusing lens group at which an "in focus" image can be formed on a film plane is predetermined in accordance with the focal length and the object distance, the focusing lens group can be moved to the predetermined position in accordance with the focal length and the object distance to form an "in focus" image. This is because a vari-focal lens can be used as a zoom lens either in an autofocus type of single-lens reflex camera or an autofocus type of compact camera. However, the focusing speed of a single-lens reflex camera is relatively slow in comparison with a compact camera. Generally, a lens which varies magnification without the movement of a focal point is called a zoom lens, and a lens in which the focal point moves in accordance with a change of magnification is called a vari-focal lens. In this disclosure, "zoom lens" includes both zoom lenses and vari-focal lenses.
There is a known zoom lens for a camera, having a first lens group with a positive focal length, a second lens group with a positive focal length, and a third lens group with a negative focal length, wherein focusing is effected by the second lens group or the third lens group. See, e.g., "Shashin Kogyo", June 1988 technical report, pages 81-86, and Japanese Patent Application No. 63-225294, filed in the name of the assignee of the present application.
Also known is a zoom lens which has a basic construction similar to the above-mentioned known zoom lens and in which the second lens group consists of two sub-groups having a negative focal length and a positive focal length, respectively, as disclosed, for example, in Japanese Patent Application No. 63-225294 mentioned above. The sub-groups of lenses are moved along different tracks during zooming and are moved together or separately during focusing. This prior Japanese patent application also discloses an alternative in which the focusing is effected by the movement of the third lens group.
These known zoom lenses are small in size and have a relatively large magnification. However, generally speaking, in a zoom lens in which the displacement of the focal point takes place in accordance with a change in magnification, the position of the zoom lens is adjusted so that no displacement of the focal point occurs when zooming is effected at an infinite object distance. The same is true in the above-mentioned prior Japanese patent application. If the number of divisions (number of steps) of the focal lengths within a zooming range is small, a deviation in focus (an amount of deviation due to divided focal lengths) at the points of division increases as the object distance decreases, due to such an adjustment of the position of the zoom lens. In a compact camera, upon focusing, the focus lens group is moved to come to a specific point of a plurality of predetermined divided points of object distance, which specific point is closest to an object distance detected by an object distance detecting device. This inevitably causes a deviation in focus (an amount of deviation due to divided object distances) at the specific point. The latter deviation of focus (an amount of deviation due to divided object distances) in addition to the former deviation of focus (an amount of deviation due to divided focal lengths) increases the degree by which the object is "out of focus" as the object distance decreases. This problem will be discussed hereinafter in more detail in connection with the present invention.